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This study focuses on neuromagnetic approaches to the detection of electrical brain activity which is difficult to detect by EEG measurements. Sources of delta waves during sleep in human subjects were investigated over both time and frequency domains. Both MEG and EEG activities were recorded simultaneously. The magnetic field components perpendicular to the surface of the head were measured using a dc SQUID with a second derivative gradiometer. The fast Fourier transform (FFT) was used to estimate the power spectrum. We observed that the frequency distributions were different for delta waves measured by MEG and by EEG. This frequency shift suggests that the sources which generate the magnetic fields are different from those which produce the EEG. The delta waves consist of many different sources with different frequencies. Some of these sources can be detected by MEG measurements, but are difficult to detect by EEG.